The present invention relates to compensation of chromatic and polarization dispersion and transmitter frequency chirp in fiber optical communication systems in order to increase the data rate. The invention is applicable to optical pulse time compression for producing short pulses from a long chirped pulses. The invention is also relevant to stretchers and compressors for distortionless amplification of short optical pulses.
One of the major applications of single-mode fiber concerns telecommunication, particularly, for trunk networks, where long-haul high-data-rate links predominate. Millions of kilometers of single-mode optical fibers are already installed throughout the world. Most of the single-mode fibers installed have high chromatic dispersion which limits the speed of intensity-modulated direct-detection optical fiber communication links. There are, currently, single-mode fibers available with less chromatic dispersion suitable for ultrahigh-speed long distance optical communication systems. However, the cost of removing the old fiber cables and installing new ones is prohibitive. Therefore, it is highly economical to increase the usable bandwidth of existing installed fibers without installing new ones.
Fiber loss and dispersion are two fundamental limiting factors in bandwidth of intensity-modulated direct-detection optical fiber communication system. With recent advent of rare earth optical amplifiers, dispersion of single-mode fiber has become the dominant limitation for ultrahigh-speed long distance optical communication systems. There are two major contributions to dispersion, chromatic and polarization. Chromatic dispersion causes pulse broadening due to unequal speed of different wavelength components of light pulse in the fiber. Polarization mode dispersion arises in single-mode fiber when the combined effects of non-symmetric internal stresses and noncircularity of the waveguide geometry created during manufacture cause the two polarization modes of the waveguide to propagate with different group velocities. Polarization mode dispersion like chromatic dispersion broadens the optical pulse in optical fibers.
Diode lasers may produce a frequency chirped optical pulse upon pulsed excitation. Each wavelength (frequency) components of a chirped optical pulse are emitted in different time, hence, causing different delays for every wavelength. Therefore, a chirped optical pulse resembles a dispersion broadened optical pulse where every wavelength component has experienced different delays. In long distance optical communication systems, dispersion degrades system performance by either limiting the maximum data rate or by requiring a shorter distance between repeaters.
To upgrade existing networks based on standard single-mode fiber, several all-optical dispersion compensation techniques have been proposed. Approaches described in U.S. Pat. Nos. 5,185,827 and 5,261,016, utilize a spatial mode converter with a dispersive waveguide having an opposite dispersive characteristic to balance the unwanted chromatic dispersion. Disadvantage of these techniques is excessive loss due to mode conversion and long length of dispersive waveguide for commercial systems.
Another method as described in U.S. Pat. No. 4,261,639, involves the interconnection of two optical fibers having appropriate lengths and having opposite group velocity dispersion characteristic so that the total dispersion in one fiber is substantially matched and canceled by the total dispersion in the connected fiber. While this is a possible solution, the length of compensating fibers are impractically long.
Use of Fabry-Perot etalon in transmissive and reflective structure for dispersion compensation is discussed in a paper by L. J. Cimini, I. J. Greenstein, and A. M. Saleh, "Optical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersion", IEEE J. of Lightwave Tech. LT-8, Page 649 (1990). The authors' technique requires continuous monitoring and tuning and appropriate means of feedback for dispersion compensation and optical equalization.
A. H. Gnauck, R. M. Jopson, and R. M. Derosier, "10-Gb / s 360-km transmission over dispersive fiber using midsystem spectral inversion", IEEE Photonics Technology Letters Vol. 5 No. 6 Page 663 (1993) utilize mid-point spectrum inversion technique by means of the nonlinear optical effects in fibers to compensate for pulse distortion. Gnauck et al and Cimini et al methods are complicated and do not lend itself to reliable practical commercial systems.
The present invention compensator has several advantages over prior arts. Unlike others, the present compensator corrects polarization as well as chromatic dispersion. The discussed prior arts either are too complicated to implement or have excessively high loss or impractically long lengths. The present invention is simple, low cost, compact, broad band, and low loss which lends itself to commercial utilization. A second application of the present invention is related to optical pulse compression. Optical pulse compression for producing picosecond and subpicosecond laser pulses has become of great interest in recent years. A traditional method to compress optical pulses is to employ a pair of diffraction gratings. E. B. Treacy. "Optical Pulse Compression with Diffraction Gratings", IEEE Journal of Quantum Electronics Vol. 5 Page 454 (1969) discusses a controllable group delay of a grating pair to overcome the inherent negative chirp of the picosecond pulses from a passively mode locked Nd-Glass laser. While grating pair method is effective, it suffers from high loss that may be unacceptable penalty for many practical applications.
As a pulse compressor for laser systems, unlike other methods, the present invention is low loss and simple and can compress positively chirped as well as negatively chirped optical pulses.
In some applications, such as shod optical pulse amplification, a pulse is intentionally broadened by means of frequency chipping. The frequency chirped pulse is amplified and, subsequently, time compressed. The reason for initial broadening of the pulse is to obviate the saturation effects in the optical amplifier, due to high optical pulse power density. The present invention is relevant in both initial pulse broadening and subsequent pulse compression in optical amplifiers.